Google has finally announced its long-awaited wireless service. Called "Project Fi", it combines access to Sprint and T-Mobile's wireless networks with the ability to make calls over Wi-Fi.

The service will let customers dynamically switch between the carriers' LTE networks and Wi-Fi networks which means calls, texts, and data will automatically be sent out via whichever service has the strongest signal. By partnering with Sprint and T-Mobile, Google will offer access to more than a million free Wi-Fi hotspots across the world.

The service starts at $20, with unlimited talk, texts, inexpensive international calls, international roaming at no additional cost and WiFi tethering. For extra $10 users can add 1GB of data. If not all of the paid data traffic is used during the current billing period, Google will refund $1 per 100 unused megabytes.

Initially, only a small number of people will be able to join the "Early Access Program" for Project Fi each week. Users who have service in their area, can request an invitation. Those who want to subscribe, should get a special SIM card and the Nexus 6 smartphone, the only phone on the market right now that supports Project Fi's "network of networks".